Georg, Joe: yes, a string is sometimes useful to see as a sequence of characters, though that's relatively uncommon. It's much more likely that a string is an atomic entity, as when it's used as an identifier, or a sequence of non-character strings, like a sequence of words, or a sequence of colon-delimited hex-encoded numbers (a MAC address). Python sure could use a standard idiom for tokenizing... but that's another issue.
And unlike many other languages, Python does not have a character object, so strings are not a sequence of characters. They are a sequence of... strings? It just doesn't make much sense. (I do think that it was the right decision to avoid character objects -- in part because I don't think it's a particularly useful metaphor)